John Z. wrote: "I think the accepted term is "casino grunge" and believe me, you don't want to know what it consists of."
I have heard the grunge comes from B,S,&T (Blood, Sweat, and Tears).
By the way, keep in mind that Charles' recommendations for cleaning are his (and probably others') OPINIONS. For example, Charles' guide states that worn chips should never be cleaned. There are others that believe that all chips should be cleaned.
There are not hard and fast rules as there are in coin collecting, where a coin should never be cleaned. No hard and fast rules are one of the good things about this hobby, in my opinion.
Thanks--Alan.